The ocean floor is the final spending frontier for the world's richest people. Journeying to see what's on the bottom aboard a personal submersible is a wretched excess guaranteed to trump the average mogul's stable of vintage Bugattis or a $38 million round-trip ticket to the International Space Station aboard a Russian rocket.

Luxury-submarine makers and salesmen from the Pacific Ocean to the Persian Gulf say fantasy and secrecy are the foundations of this nautical niche industry.

"Everyone down there is a wealthy eccentric," says Jean-Claude Carme, vice president of marketing for U.S. Submarines Inc., a Portland, Ore.-based bespoke submarine builder.

Who owns the estimated 100 luxury subs carousing the Seven Seas is mostly a mystery.

"Not really supposed to talk about the sub, but it's a fancy one," says Fred Rodie, an engineer who designed Allen's sub at Olympic Tool & Engineering Inc. in Shelton, Wash.

"If I told you, I'd have to shoot you," says Bruce Jones, president and founder of U.S. Submarines, about his clients.

Jones, the 50-year-old son of a marine-construction engineer, built his first diesel- and battery-powered sub in 1993. Every sales contract since then has included a confidentiality clause to protect the buyer's identity.

Herve Jaubert, a former French Navy commando, swapped his cutlass for a screwdriver in 1995 to build his first luxury sub. Now CEO of Exomos, a Dubai-based custom-sub maker, Jaubert takes a more romantic view of the work: "I'm a poet who builds submersible yachts for rich people."

"Spending $80 million for a boat that goes underwater in a market where one that doesn't costs $150 million is a deal," Jones says. "Our Phoenix 1000 is a 4-story tall blend of a tourist and military sub."

The ultimate war submarine, the U.S. Navy's Virginia-class New Attack Submarine, costs $2.4 billion and carries 16 Tomahawk cruise missiles. Jones says the most dangerous projectile aboard the Phoenix 1000 is a champagne cork.

"I build luxury-delivery systems for people who have more money than they know what to do with," Jones says, walking near Idaho's Lake Pend Oreille, site of the Farragut Naval Submarine Training Station.

Jaubert's 10-passenger sub costs $15 million. A gymnasium is optional. U.S. Submarines' mid-size model is the $25 million Seattle 1000, a 3-story vessel with five staterooms, five bathrooms, two kitchens, a gym, a wine cellar and a 30-by-15-foot observation portal. It has a range of 3,000 nautical miles.

"The one thing I won't make for anyone is a yellow submarine," Jones says.

The 40-foot sub owned by Microsoft's Allen came with a $12 million sticker and enough extras to remain submerged for a week. Its color: yellow.

"The Proteus is an underwater bus," Jaubert says. "It's more fun in the Stingray, drives like a Ferrari."

Jaubert says one of the dangers shared by members of this underwater fraternity of the super-rich is being blown to smithereens by depth charges.

"Side sonar scanners are mistaken for torpedo tubes," the 50-year-old engineer says, slapping the blue hull of a three-seat, $350,000 "sport luxury model" under construction. "Government agencies visit to see if there are torpedoes aboard our boats. Owners are supposed to let authorities know when they're in the area. They don't, and it causes problems."